In a message dated 25/07/2009, brett3nt@............. writes:
 
>     You CAN'T cancel out tilt variations in  vertical sensors by just 
using two identical instruments!
>   You CAN do it using a seismometer AND a pure  tiltmeter.



Why  not? Can't you just orient them in opposite directions? Vertical  
motions should add, while tilt accelerations will subtract. That's what we do  
when we want to evaluate the magnitude of tilt-rotation noise on Dave's  
verticals.
Hi Brett,
 

That won't work!
 
    The vertical force change due to a tilt theta is  mgCos(theta) = 1 - 
theta^3 / !3 + 
 
    This is HIGHLY non linear with angle. Moreover, you get  the same 
negative polarity force change for tilts in ANY orientation in the  horizontal 
plane. 
 
    You might want cross check your assumptions about  tilt rotation noise?


Obviously DC tilts don't necessarily cancel, but I wouldn't expect  that to 
be the primary problem.  Come to think of it, wouldn't you even  be able to 
cancel steady-state tilts if you adjusted one vertical instrument  slightly 
off-center in one direction and the opposing one the same amount in  the 
opposite direction?


Not in general. If you set up two sensors with  identical offsets in 
opposite directions, they will both still produce a tilt  error if the tilt 
direction is at right angles to the line joining the  centres of the two 
instruments.
You need triaxial sensors to resolve this, but you still won't know if  an 
effect is due to a tilt, or to a horizontal acceleration. You need a pure  
tiltmeter to resolve this hoary ancient problem - a liquid level meter WON'T  
do!
 
    Regards,
 
    Chris





In a message dated 25/07/2009, brett3nt@............. writes:

>     You CAN'T cancel out tilt variation=
s in 
  vertical sensors by just using two identical instruments!
>&n=
bsp; 
     You CAN do it using a seismometer AND a pure 
tiltmeter.

Why 
  not? Can't you just orient them in opposite directions? Vertic=
al 
  motions should add, while tilt accelerations will subtract. That's what=
 we do 
  when we want to evaluate the magnitude of tilt-rotation noise on Dave's=
 
  verticals.
Hi Brett,

 
    That won't work!
 
    The vertical force change due to a tilt theta=
 is 
mgCos(theta) =3D 1 - theta^3 / !3 + 
 
    This is HIGHLY non linear with angle. Moreover, yo=
u get 
the same negative polarity force change for tilts in ANY orientation=
 in the 
horizontal plane. 
 
    You might want cross check your assumptions=
 about 
tilt rotation noise?
Obviously DC tilts don't necessarily cancel, but I wouldn't=
 expect 
  that to be the primary problem.  Come to think of it, wouldn't you=
 even 
  be able to cancel steady-state tilts if you adjusted one vertical instru=
ment 
  slightly off-center in one direction and the opposing one the same amoun=
t in 
  the 
opposite direction?


    Not in general. If you set up two sensors wit=
h 
identical offsets in opposite directions, they will both still produce a=
 tilt 
error if the tilt direction is at right angles to the line joining th=
e 
centres of the two instruments.
You need triaxial sensors to resolve this, but you still won't=
 know if 
an effect is due to a tilt, or to a horizontal acceleration. You need a pu=
re 
tiltmeter to resolve this hoary ancient problem - a liquid level meter WON=
'T 
do!
 
    Regards,
 
    Chris